The present invention relates to a zoom lens, and in particular, to a small-sized 2-group zoom lens which has less restriction of back focus and is suitable for a camera of a lens shutter type, and to a camera.
As a type of a zoom lens, there has been a 2-group zoom lens. The 2-group zoom lens is composed of a first lens group having positive refracting power and a second lens group having negative refracting power both arranged in this order from an object, and is simple in structure and has a short total length. Due to this, the 2-group zoom lens is used widely for a camera such as a lens shutter type camera.
The 2-group zoom lens is classified into a negative lens preceding type and a positive lens preceding type, depending on whether the first lens is positive or negative. The positive lens preceding type is suitable for focal length range in a telephoto system because it is easy to make a telephoto ratio to be small.
However, it has been difficult to make the 2-group zoom lens to be a compact zoom lens wherein aspects of cost are taken into consideration, because of the reason that the 2-group zoom lens of a positive lens preceding type has many constituting lenses and has many glass lens structures.
The invention has been achieved in view of the problems stated above, and its object is to provide a zoom lens which is corrected in terms of various aberrations, and is compact in structure, low in cost, excellent in image-forming performance and is suitable for a lens shutter type camera.
Further, the invention makes it possible to correct chromatic aberration satisfactorily even when many plastic lenses are used and to compensate changes in focal length and back focus caused by temperature fluctuation.
In a conventional 2-group zoom lens employing a plastic lens, there has been a problem that the number of structural lenses is large when trying to obtain a large zoom ratio, resulting in complicated structure and an increase in cost. In particular, when many plastic lenses are used in a 2-group zoom lens, for the purpose of low cost, a large ratio of magnification tends not to be obtained, compared with an occasion wherein many glass lenses which are relatively expensive are used, resulting in poor balance between cost and function.
Another object of the invention is to provide a zoom lens which is of low cost, corrected satisfactorily in terms of various aberrations even when a zoom ratio is large, and is suitable for a lens shutter type camera.
The objects stated above can be attained by either one of the following structures.
Structure (1-1) A zoom lens to form an image of an object with variable magnification, comprises:
a first lens group having a positive refracting power, and
a second lens group positioned closer to the image than the first lens group and having a negative refracting power,
wherein, when the magnification is changed, a distance between the first lens group and the second lens group is changed,
wherein the first lens group comprises a 1-1 lens, a 1-2 lens and a 1-3 lens aligned in this order from the object, and
wherein the zoom lens comprises at least two plastic lenses.
Structure (1-2) In the zoom lens of Structure (1-1), the first lens group does not comprise a 1-4 lens and the second lens group comprises a 2-1 lens and a 2-2 lens aligned in this order from the object, and the 1-1 lens is a plastic lens, the 1-3 lens has a positive refracting power and the 2-2 lens has a negative refracting power.
Structure (1-3) In the zoom lens of Structure (1-2), the 1-2 lens has a negative refracting power and following formulas are satisfied:
1.50xe2x89xa6n1-3xe2x89xa61.90
1.50xe2x89xa6n2-2xe2x89xa61.90
where n1-3 is a refractive index of the 1-3 lens, and n2-2 is a refractive index of the 2-2 lens.
Structure (1-4) In the zoom lens of Structure (1-2), the 1-2 lens is a plastic lens, at least four surfaces of the lenses in the entire system of the zoom lens is an aspherical surface, and a following formula is satisfied:
1.50xe2x89xa6n2-2xe2x89xa61.90
where n2-2 is a refractive index of the 2-2 lens.
Structure (1-5) In the zoom lens of Structure (1-2), the 1-2 lens is a plastic lens and following formulas are satisfied:
xe2x80x831.50xe2x89xa6n1-3xe2x89xa61.90
1.50xe2x89xa6n2-2xe2x89xa61.90
where n1-3 is a refractive index of the 1-3 lens, and n2-2 is a refractive index of the 2-2 lens.
Structure (1-6) In the zoom lens of Structure (1-2), the 1-1 lens is a plastic lens having a positive refractive power, the 1-2 lens is a plastic lens, the 1-3 lens is a meniscus shaped glass lens having a positive refractive power wherein an image side surface of the meniscus shaped glass lens is concave, and the 2-2 lens is a plastic lens having a negative refractive power.
Structure (1-7) In the zoom lens of Structure (1-2), the 2-1 lens is a glass lens.
Structure (1-8) In the zoom lens of Structure (1-2), the 1-4 lens has at least one aspherical surface and following formulas are satisfied:
0.64xe2x89xa6fW/Dxe2x89xa61.0
0.50xe2x89xa6|fRC|/fWxe2x89xa60.80
where fW is a focal length of the zoom lens at a wide angle end, D is a length of a diagonal line of an image plane, and fRC is a focal length of the second lens group.
Structure (1-9) In the zoom lens of Structure (1-2), the 1-1 lens is a positive refracting power and has at lease one aspherical surface, the 2-1 lens is a plastic lens and has at lease one aspherical surface, and a following formula is satisfied:
xe2x88x925xe2x89xa6(xcfx86P+xcfx864)xc2x7fT2/FTxe2x89xa65
where xcfx86P is a power of a plastic lens of the first lens group, xcfx864 is a power of the 2-1 lens, fT is a focal length of the zoom lens at a telephoto end and FT is a F-number of the zoom lens at a telephoto end.
Structure (1-10) In the zoom lens of Structure (1-1), the first lens group comprises a 1-4 lens which is a glass lens having a positive refracting power and the second lens group comprises a 2-1 lens and a 2-2 lens, and wherein the first lens group comprises at least one plastic lens having a negative refracting power and at least one plastic lens having a positive refracting power.
Structure (1-11) In the zoom lens of Structure (1-1), the magnification ratio is 2.3 or more, the first lens group comprises a 1-4 lens and the second lens group comprises a 2-1 lens and a 2-2 lens, and wherein the first lens group comprises at least one plastic lens having a negative refracting power and at least one plastic lens having a positive refracting power.
Structure (1-12) In the zoom lens of Structure (1-1), the first lens group is separated by the largest air space into a 1-a lens sub-group and a 1-b lens sub-group, and the second lens group comprises a 2-1 lens and a 2-2 lens, and wherein the 1-a lens sub-group comprises at least two lenses comprising at least one plastic lens having a negative power and the 1-b lens sub-group comprises at least two lenses comprising at least one plastic lens having a positive power
Structure (1-13) In the zoom lens of Structure (1-1), the first lens group comprises at least one glass lens and at least three plastic lenses and the second lens group comprises at most two lenses comprising at least one plastic lens.
Structure (1-14) In the zoom lens of Structure (1-1), the first lens group comprises at least two glass lenses and at least two plastic lenses, the second lens group comprises a 2-1 lens and a 2-2 lens and a following formula is satisfied:
xe2x88x920.50xe2x89xa6|fRC|/f2-1xe2x89xa60.20
where fRC is a focal length of the second lens group and f2-1 is a focal length of the 2-1 lens.
Structure (1-15) In the zoom lens of Structure (1-1), all of the plastic lenses of the zoom lens have a saturated water absorption of 1.2% or less.
Structure (1-16) In the zoom lens of Structure (1-1), all of the plastic lenses of the zoom lens have a refractive index of 1.5 or more.
Structure (1-17) In the zoom lens of Structure (1-1), the 1-1 lens is a plastic lens having a positive refracting power.
Structure (1-18) A camera to photograph an image of an object with variable magnification, comprises:
a zoom lens having
a first lens group having a positive refracting power, and
a second lens group positioned closer to the image than the first lens group and having a negative refracting power,
wherein when the magnification is changed, a distance between the first lens group and the second lens group is changed,
wherein the first lens group comprises a 1-1 lens, a 1-2 lens and a 1-3 lens aligned in this order from the object, and
wherein the zoom lens comprises at least two plastic lenses.
Structure (1-19) In the camera of Structure (1-18), the first lens group does not comprise a 1-4 lens and the second lens group comprises a 2-1 lens and a 2-2 lens aligned in this order from the object, and
wherein the 1-1 lens is a plastic lens, the 1-3 lens has a positive refracting power and the 2-2 lens has a negative refracting power.
Further, the objects stated above may be attained by either one of the following preferable structures.
Structure (2-1) A zoom lens composed of a first lens group having positive refracting power and a second lens group having negative refracting power both arranged in this order from an object, in which a magnifying power is varied by changing a distance between the first lens group and the second lens group, wherein the first lens group is composed of three lens elements including a 1-1 lens, a 1-2 lens and a 1-3 lens arranged in this order from an object and of a diaphragm located to be closer to images formed by the lenses, and the 1-1 lens is a plastic lens, the 1-2 lens has negative refracting power and the 1-3 lens has positive refracting power, while, the second lens group is composed of two lens elements, one is a 2-1 lens and the other is a 2-2 lens having negative refracting power wherein both lenses are arranged in this order from an object, and the following conditions are satisfied.
1.50xe2x89xa6n1-3xe2x89xa61.90xe2x80x83xe2x80x83(1)
xe2x80x831.50xe2x89xa6n2-2xe2x89xa61.90xe2x80x83xe2x80x83(2)
wherein
n1-3: Refractive index of 1-3 lens
n2-2: Refractive index of 2-2 lens
Structure (2-2) In the zoom lens according to Structure (2-1), the 1-3 lens is a glass lens.
Structure (2-3) In the zoom lens according to Structure (2-1) or (2-2), at least one surface of 2-1 lens is an aspherical surface and the following conditions are satisfied.
0.64xe2x89xa6fW/Dxe2x89xa61.0n1-3xe2x80x83xe2x80x83(3)
0.50xe2x89xa6|fRC|/fWxe2x89xa60.80xe2x80x83xe2x80x83(4)
wherein
fW: Focal length at wide angle end
D: Length of a diagonal line of an image plane
fRC: Focal length of the second lens group
Structure (2-4) In the zoom lens according to Structure (2-1), the 1-1 lens is a plastic lens which has positive refracting power and employs an aspherical surface on at least one of its surfaces, while the 2-1 lens is a plastic lens which employs an aspherical surface on at least one of its surfaces, and satisfies the following condition.
xe2x88x925xe2x89xa6(xcfx86P+xcfx864)xc2x7fT2/FT less than 5xe2x80x83xe2x80x83(5)
wherein
xcfx86P: Power of a plastic lens in the 1-1 lens
xcfx864: Power of the 2-1 lens
fT: Focal length at telephoto end
FT: F-number at telephoto end
Structure (2-5) A zoom lens composed of a first lens group having positive refracting power and a second lens group having negative refracting power both arranged in this order from an object, in which a magnifying power is varied by changing a distance between the first lens group and the second lens group, wherein the first lens group is composed of three lens elements including a 1-1 lens, a 1-2 lens and a 1-3 lens arranged in this order from an object and of a diaphragm located to be closer to images formed by the lenses, and the 1-1 lens and the 1-2 lens are represented by a plastic lens, the 1-3 lens has positive refracting power while, the second lens group is composed of two lens elements one is a 2-1 lens and the other is a 2-2 lens having negative refracting power wherein both lenses are arranged in this order from an object, and an aspherical surface is used on at least four surfaces and the following conditions are satisfied.
1.50xe2x89xa6n2-2xe2x89xa61.90xe2x80x83xe2x80x83(6)
wherein
n2-2: Refractive index of the 2-2 lens
Structure (2-6) In the zoom lens according to Structure (2-5), the 1-3 lens is a glass lens.
Structure (2-7) In the zoom lens according to Structure (2-5) or (2-6), at least one surface of the 2-1 lens is an aspherical surface and the following conditions are satisfied.
0.64xe2x89xa6fW/Dxe2x89xa61.0xe2x80x83xe2x80x83(7)
0.50xe2x89xa6|fRC|/fWxe2x89xa60.80xe2x80x83xe2x80x83(8)
wherein
fW: Focal length at wide angle end
D: Length of a diagonal line of an image plane
fRC: Focal length of the second lens group
Structure (2-8) In the zoom lens according to either one of Structures (2-5)-(2-7), the 1-1 lens has positive refracting power, the 1-2 lens has negative refracting power, while the 2-1 lens is a plastic lens and the following condition is satisfied.
Incidentally, the length D of a diagonal line of an image plane is represented by the following formula:
D=2xc3x97fWxc3x97tan xcfx89W
Where xcfx89W is the largest half angle of view at the wide angle end.
xe2x80x83xe2x88x925xe2x89xa6(xcfx861+xcfx862+xcfx864)xc2x7fT2/FTxe2x89xa65xe2x80x83xe2x80x83(9)
wherein
xcfx86i: Power of i-th lens
fT: Focal length at telephoto end
FT: F-number at telephoto end
Structure (2-9) A zoom lens composed of a first lens group having positive refracting power and a second lens group having negative refracting power both arranged in this order from an object, in which a magnifying power is varied by changing a distance between the first lens group and the second lens group, wherein the first lens group is composed of three lens elements including a 1-1 lens, a 1-2 lens and a 1-3 lens arranged in this order from an object and of a diaphragm located to be closer to images formed by the lenses, and the 1-1 lens and the 1-2 lens are represented by a plastic lens, the 1-3 lens has positive refracting power, while, the second lens group is composed of two lens elements one is a 2-1 lens and the other is a 2-2 lens having negative refracting power wherein both lenses are arranged in this order from an object, and the following conditions are satisfied.
1.50xe2x89xa6n1-3xe2x89xa61.90xe2x80x83xe2x80x83(10)
1.50xe2x89xa6n2-2xe2x89xa61.90xe2x80x83xe2x80x83(11)
wherein
n1-3: Refractive index of 1-3 lens
n2-2: Refractive index of 2-2 lens
Structure (2-10) In the zoom lens according to Structure (2-9), the 1-3 lens is a glass lens.
Structure (2-11) In the zoom lens according to Structure (2-9) or (2-10), at least one surface of 2-1 lens is an aspherical surface and the following conditions are satisfied.
0.64xe2x89xa6fW/Dxe2x89xa61.0xe2x80x83xe2x80x83(12)
0.50xe2x89xa6|fRC|/fWxe2x89xa60.80xe2x80x83xe2x80x83(13)
wherein
fW: Focal length at wide angle end
D: Length of a diagonal line of an image plane
fRC: Focal length of the second lens group
Structure (2-12) In the zoom lens according to either one of Structures (2-9)-(2-11), the 1-1 lens has positive refracting power and at least one surface of which is an aspherical surface, the 1-2 lens has negative refracting power and at least one surface of which is an aspherical surface, while the 2-1 lens is a plastic lens and the following condition is satisfied.
xe2x88x925xe2x89xa6(xcfx861+xcfx862+xcfx864)xc2x7fT2/FTxe2x89xa65xe2x80x83xe2x80x83(14)
wherein
xcfx86i: Power of i-th lens
fT: Focal length at telephoto end
FT: F-number at telephoto end
Each conditional expression will be explained here.